Global water-nutrient-salinity-energy nexus in lettuce production: From open-field irrigation to closed-loop hydroponics in greenhouses

Hydroponic greenhouse production is a proven approach to enhancing resource efficiency and overcoming limitations related to land availability, soil degradation, and climate variability. However, assessing the benefits and challenges of transitioning to greenhouses remains underexplored. This study provides a global assessment of the water-nutrient-salinity-energy nexus in lettuce production, spanning from soil-based open-field systems to closed-loop hydroponic greenhouse technologies. The study examines soil-water interactions in conventional open-field agriculture and highlights critical challenges, including soil salinization and nutrient leaching inherent to open-field farming. While transitioning to closed-loop hydroponics in greenhouses offers a potential solution to these issues, it comes with a considerable trade-off in the form of elevated energy demands for greenhouse operations. This study quantifies both the water demand for hydroponics and the energy required for heating and cooling, while also evaluating the contribution of harvested rooftop rainfall. Adopting closed-loop hydroponic systems in greenhouses can reduce water consumption for lettuce production by 6% to over 90%, while eliminating risks of soil salinization and groundwater contamination. Harvested rooftop rainfall can meet 100% of the hydroponics water demand in some locations, such as Bangladesh, during peak monsoon months (June–August). However, globally, the total energy demands for heating and cooling range from 39 to 123 MJ/kg, depending on climate. Furthermore, the minimum annual energy requirement for greenhouse operation was found to be 1854 MJ/m2 in Cwc climates (temperate, dry winters, and cold summers), with energy distribution favoring heating (62%) over cooling (38%). In India, specific water use (evapotranspiration and water for evaporative cooling) decreased by 93%, from 1146 L/kg in open-field furrow systems to 85 L/kg in closed-loop hydroponics, with an energy demand of 66 MJ/kg. These findings offer comprehensive, global-scale insights across 30 Köppen–Geiger climate zones and the 15 leading lettuce-producing countries, providing a foundation for policymakers to plan future agricultural development.